For more than a decade it has been known that the vascular endothelium releases a labile factor involved in the relaxation of smooth muscle when stimulated by a variety of vasodilator substances such as acetylcholine (1,2). The identity of this mediator, known originally as endothelium-derived relaxing factor, remained elusive until 1987, when it was demonstrated that nitric oxide gas (NO) had biological properties identical to those of endothelium-derived relaxing factor and that it is released from endothelial cells in quantities sufficient to explain its actions (3). These findings were independently confirmed by other groups (4, 5).
The next phase in the biological history of this simple "inorganic" molecule was the finding that the amino acid L-arginine is the substrate for the enzyme NO synthase. This enzyme generates NO from the terminal guanidino nitrogen atoms of L-arginine (6) through a process in which molecular oxygen is also incorporated (7). The identification of L-arginine analogs such as NG-monomethyl-L-arginine (L-NMMA) as specific inhibitors of NO biosynthesis (8, 9) was a crucial step in understanding this biochemical pathway.
With the availability of these NO synthase inhibitors, the biological sigmfmnce of this elementary molecule could be more fully appreciated. Thus early in uiuo experiments in the rabbit and rat ( 1 0 , l l ) demonstrated that the systemic hypotensive action of acetylcholine could be attenuated by L-NMMA administration; this supported findings obtained using isolated vascular tissue (8, 9). Of fundamental importance was the realization that endogenous NO has a profound influence on the cardiovascular system because L-NMMA substantially elevates systemic arterial blood pressure (10-12). The specificity of these effects of L-NMh4.A was demonstrated by the failure of its enantiomer D-NMMA to exert such cardiovascular actions, and the actions of L-NMMA were abolished by concurrent administration of L-arginine but not D-arginine. Studies in human subjects confirmed the importance of NO in the regulation of vascular tone because infusion of L-NMMA into the brachial artery caused vasoconstriction and attenuated the vasodilatation induced by acetylcholine (13).
Further studies in uiuo established a role for endogenous NO in the regulation of blood flow in the gastric mucosa and splanchnic circulation (14,15